Method for locating the remaining recoverable mineral reserves during solution mining

ABSTRACT

The present invention relates to a method for the solution mining of a mineral from a subterranean formation. More specifically, the invention relates to a method which enhances significantly the recovery of a mineral from a subterranean formation via solution mining utilizing an oxidant and leach solution with an injection and production well. The method comprises ceasing oxidant injection via the injection well and measuring the change in mineral concentration in the pregnant solution recovered via the production well.

Generally, known methods for solution mining of a mineral in situutilize an acid or alkaline leach solution for the dissolution of themineral. An oxidant is injected into the formation along with the leachsolution. The mineral is leached from the formation and recovered from aproduction well via a pregnant leach solution. Various procedures forrecovering the mineral from the pregnant leach solution are well known,such as ion exchange.

The method of the present invention is particularly suitable for theleaching of uranium; however, my invention is not so limited. Thefollowing description of the present invention will be applied touranium leaching; however, it is apparent that it is applicable toleaching other mineral values such as copper, nickel, molybdenum,rhenium and selenium where similar problems are encountered.

Although acid leaching solutions can be used in some formations, onlyalkaline leaching solutions can practically be used where the particularformation contains significant quantities of acid-consuming gangue.

It is well known that to recover uranium from an underground ore body,it is necessary to convert the relatively insoluble tetravalent state ofuranium in the ore to the solubilizable hexavalent state. When using analkaline leach solution, the dissolution of the uranium in solutionoccurs in two steps. The first step involves the oxidation of uranium byadsorbing oxygen and the second is the dissolution of the oxidizeduranium in the solution.

During the early stages (when the ore body is in a reduced state) of aleach operation utilizing alkaline solutions of ammonium carbonate,sodium carbonate, potassium carbonate and their respective bicarbonatesin conjunction with the typical oxidants of air, oxygen, and hydrogenperoxide, a portion of the uranium that is oxidized and dissolved nearthe injection well is reduced and precipitated in the more reducedregions of the formation between the injection well and the productionwell. Through this action the oxidized region of the formation isdepleted of uranium and the reduced region of the formation becomesenriched as the leach operation continues. Therefore, this process ofoxidation and dissolution followed by partial reduction andprecipitation continues as the formation becomes progressively oxidized,whereby the region in the immediate vicinity of the production wellbecomes progressively enriched. As a result, the uranium is depletedfrom a zone in the formation far more quickly than the oxidant consuminggangue species present therein.

Therefore, as the uranium oxidation front recedes from the injectionwell, the oxidant available for uranium oxidation decreases. This causesa slower dissolution of uranium and a lower maximum concentration ofuranium in solution as the depleted zone moves through the formation.The continued injection of oxidant and leach solution through aninjection well away from which the uranium oxidation front has moved iswasteful, since most of the injected oxidant reacts with gangue materialand leach solution contacts little oxidized uranium. Fluid produced froma production well in exclusive communication with such an injection wellcontains little or no uranium. In a multi-well configuration, the fluiddilutes the uranium concentration from other wells and thereby decreasesthe efficiency of the operation. To conduct a solution mining operationefficiently and provide for maximum uranium recovery, it is necessary tofocus the flow of oxidant and leachant on the remaining uranium reservesand to minimize the flow through depleted or barren areas of theformation. To achieve this, the location of the remaining uraniumreserves must be known. Therefore, there is needed a method whereby theremaining reserves in a formation containing a mineral such as uraniumcan be located and recovered with a leach solution without beingaccompanied by excessive losses of oxidant and leach solution.

Therefore, it is an object of the present invention to provide a methodfor the solution mining of a mineral from a subterranean formation,applicable generally to minerals requiring oxidation to be leached andto both acid and alkaline leach solutions.

A further object of the present invention is to provide a method for thesolution mining of uranium.

It is an additional objective of the present invention to provide amethod for locating remaining reserves recoverable by solution miningfrom subterranean deposits.

Other objects, aspects, and the several advantages of the presentinvention will become apparent upon a further reading of this disclosureand the appended claims.

It has now been found that the objects of the present invention can beattained in the solution mining of a mineral from a subterraneanformation containing same where an injection and production well aredrilled and completed within said formation, a leach solution and anoxidant are injected through the injection well into the formation todissolve the mineral and recover it via a production well, bytemporarily ceasing the injection of oxidant and measuring the change inmineral concentration in the solution recovered via the production well.

In the operation of the present invention to locate remainingrecoverable uranium, the injection of oxidant into a given injectionwell is stopped while continuing leach solution injection for a periodof time required to measure the change in uranium concentration in thepregnant leach solution recovered via the production well. By measuringthe uranium concentration in the pregnant solution without oxidantinjection, one can determine the relative uranium production contributedby a given injection well. Thus, the remaining recoverable uranium islocated and steps taken to most efficiently utilize oxidant and leachsolution to recover same.

Of course, the period of time which oxidant injection should be stoppedwill vary from formation to formation. Suitable dye tests can be made todetermine more specifically the actual time needed to yield a goodobservation of the location of the reserves remaining.

The following illustrations will show the effective operation of thepresent invention.

1. When the ceasing of oxidant injection into an injection well producesno change in the uranium concentration of the pregnant solutionrecovered from an adjacent production well, the recoverable uranium inthe area is completely oxidized. Thus, oxidant injection should bepermanently stopped because continued injection merely oxidizes thegangue material present therein. Leach solution should continue to beinjected to recover the oxidized uranium.

2. When the individual interruption of oxidant injection into fouroffset injection wells centered by a production well shows that three ofthe wells provide essentially all of the uranium production, therecoverable uranium has been depleted near the fourth well. The fourthinjection well should be abandoned for efficient operation.

3. By the testing of four offset injection wells centered by aproduction well via oxidant cessation, it is found that the fourinjection wells are contributing to the production of uranium in theorder of 5, 15, 30 and 50% corresponding to each of the wells. Since theconcentration of uranium in the pregnant solution is proportional to theremaining recoverable reserves, this shows that the latter injectionwell has the largest amount of remaining recoverable reserves. Byutilizing the information obtained, an efficient strategy of finalstripper stage of operation can be developed.

Therefore, through the utilization of the present invention, therecovery of uranium via in situ leaching processes can be enhancedsignificantly by most effectively using the oxidant and leach solutionto recover remaining reserves present in the formation.

In addition, the present invention provides direct measurements of thepattern performance under normal operating conditions, since the streamlines between injection and production wells remain constant during itsutilization.

Having thus described my invention, I claim:
 1. A method for locatingthe remaining mineral reserves recoverable via solution mining from asubterranean formation containing said mineral in which an injection andproduction well are drilled and completed within said formation, leachsolution and an oxidant are injected through said injection well intosaid formation to dissolve said mineral, and said dissolved mineral isrecovered via said production well, which comprises temporarily ceasingsaid injection of oxidant and measuring the change in mineralconcentration in the solution recovered via said production well.
 2. Themethod of claim 1 wherein said mineral is selected from the groupconsisting of copper, nickel, molybdenum, rhenium, selenium and uranium.3. The method of claim 1 wherein said leach solution is acidic innature.
 4. The method of claim 3 wherein said acid leach solution isselected from the group consisting of hydrochloric and sulfuric acid. 5.The method of claim 1 wherein said leach solution is alkaline in nature.6. The method of claim 5 wherein said alkaline leach solution is anaqueous solution of one or more salts selected from the group consistingof ammonium carbonate, sodium carbonate, potassium carbonate and theirrespective bicarbonates.
 7. The method of claim 1 wherein said oxidantis selcted from the group consisting of air, oxygen and hydrogenperoxide.
 8. The method of claim 1 wherein said injection and saidproduction well are arranged in a five spot pattern having oneproduction well centered between four offset injection wells.